There are a number of applications and devices, which require for operation light that spans a broad wavelength spectrum. Since individual sources of light are only efficient in spanning portions of the broad wavelength spectrum needed, compound or composite light sources have been designed for these applications and devices. In particular, when the broad wavelength spectrum includes the ultra-violet (UV) and visible wavelengths two sources are usually combined into a broadband source covering this spectrum. This situation is encountered, for example, in scatterometry applications and devices where broad band light obtained from the compound source is used for determining optical properties of various materials as well as film thicknesses, geometric profiles of gratings and contact holes as well as many other non-destructive small-scale measurements.
The combining of individual light sources to produce a single broadband output beam that satisfies the constraints imposed by the measurement method is a complex task. To accomplish this task, light from a first source and light from a second source are combined with appropriate beam shaping and combining optics. Optics for efficiently capturing and shaping light from a single source are well-known. In fact, already in the electric lamp of U.S. Pat. No. 1,961,964 to Dodge a light shaping concave reflector is used for directing the light from a light source. Furthermore, U.S. Pat. No. 2,064,252 to Fortney teaches the use of incandescent lamp with a back reflector mirror to collimate the beam of light and increase illumination for use in automobile industry.
Of course, in building a compound light source, it is not sufficient to just efficiently capture light from a single source. It is also crucial that the light from the individual sources be efficiently combined. Here the prior art teaches to combine two sources covering the visible and the UV spectra, respectively. For example, in U.S. Pat. No. 4,611,143 Shimaza et al. teach the use of a deuterium lamp and a tungsten lamp in one sealed envelope to obtain a single broadband source. The tungsten lamp part of the system consists of a sealed envelope that forms a convex lens that focuses the visible light into the ARC aperture of the UV light generator. A concave mirror arranged behind the tungsten lamp reflects light emitted behind the filament back to the tungsten filament so as to maximize the quantity of light in the form of a beam at the location of the convex lens.
A more specific illumination system especially designed for measuring film thickness in described in U.S. Pat. No. 5,686,993 to Kokubo et al. Here, the inventors have combined halogen and deuterium lamps to form a broadband source that uses an off axis ellipsoidal reflector and half mirror to direct the combined broadband beam at the surface of the sample. The system includes a glass rod that corrects the wavelength dependence of the deuterium and halogen lamps, even when an eclipse in reflected light due to inclination of the sample decreases the energy of the reflected light. As a result, the spectral distribution of the reflected light entering a spectrometer unit remains almost unchanged.
Operating on broadband light beams places stringent constraints on the types of optics used. In particular, refractive optics such as lenses, tend to perform poorly and all reflective optics may be preferred for such applications. Although not a dual-source, teachings on how to use reflective optics in broadband spectroscopic ellipsometry are found in U.S. Pat. No. 5,910,842 to Piwonka-Corle et al. Here an Xe arc lamp is used as the source and two off-axis parabolic mirrors are employed to focus the light into a fiber optics. Further specific improvements to guiding light from sources such as deuterium lamps are found in U.S. Pat. No. 5,972,469 to Curtis, who teaches the use of an improved baffle to direct the light discharged from a deuterium lamp to increase light intensity and directionality and obtain reduction in ring formation.
More recent work has also focused on combining more than two sources and achieving a flattened intensity distribution in broadband beams for optical measurements in ellipsometers, spectrophotometers and polarimeters. For example, in U.S. Pat. No. 6,268,917 Johs teaches combining Xe, deuterium and quartz halogen lamps to obtain near flat broadband beam for material system investigation. Still other approaches to combining two sources for coaxial illumination for optical metrology are found, e.g., in U.S. Pat. No. 6,862,090 to Chen et al. and in U.S. Patent Application 2003/0020912 to Norton et al.
Unfortunately, none of the prior art solutions provides for a broadband light source that efficiently combines sources, provides a reasonably flat spectrum and achieves suitably high intensity at a correspondingly small spot size for examination of samples with small features. In fact, although today a 50×50 μm2 test pad is used as a standard size in semiconductor manufacturing devices, as the packing density increases and features are shrinking there is increased demand for smaller test pads. Conventionally, the spot size reduction is accomplished by application of refractive lenses. However, in a broadband system these lenses add complication such as aberration and require frequent alignments. Moreover, addition of refractive lenses frequently contributes to significant reduction in light intensity.